Telephone system



Jan. 23, 1940. R. H. HERRICK 2,137,903

TELEPHONE SYSTEM Filed May 21, 1938 7 Sheets-Sheet 2 [X3 mmwu R84 wmwu R K AmN.u N w u m l 6am E R (Q0 7 um N m 8 E v mmmui L WmN R W m R Y B ATTORNEYS.

Jan. 23, 1940.

R. H. HERRICK 2,187,903

TELEPHONE SYSTEM Filed May 21, 1938 '7 Sheets-Sheet 5 A TTORNEYS.

Jan. 23, 1940.

R. H. HERRIC K TELEPHONE SYSTEM Filed May 21, 1938 7 Sheets-Sheet 5 ROSWEI L H. HERRICK BY PM ATTORNEYS.

Jan. 23, 1940. R. H. HERRICK TELEPHONE SYSTEM Filed May 21, 1938 7 Sheets-Sheet 6 Jill mKWU V mu INVENTOR. ROSWELL H.HERR|CK M M A TTORNEYS Jan. 23, 1940. R. H. HERRICK TELEPHONE SYSTEM Filed May 21, 1938 7 Sheets-Sheet T INVENTOR. ROSWELL H. HERRICK VM/ ATTORNEYS.

Patented Jan. 23, 1940 UNITED STATES PATENT OFF-lCE I TELEPHONE SYSTEM Roswell Harry Herrick, Oak Park, 111., assignor to Associated Electric Laboratories, Inc., Chicago,

111., a corporation of Delaware, I

Application 'May 21, 1938, Serial No. 209,197

18 Claims. (01. can)" The present invention relates to telephone sy terns and more particularly to improved telephone apparatus for providing simplified and "more direct calling facilities between the telephone substation of an important executiveor oflicial and the telephone substations of subordinates: who are frequentlycalled from the executive telephone substation. In applicant's copending application, Serial No. 207,433, filed May 12, 1938,v there is disclosed an improved telephone system which is so arranged that the delay incident to the operation of a manual system or the labor incident to the operation of the calling devicein automatic systems is obviated. "This system 'is so arranged that calls to a number of telephones may be initiated at the special substation either by using aconventional dial operated calling device or by the simple expedient of selectively actuating calling keys individually corresponding to the telephones which are frequently called. The disclosed arrangement is particularly adapted to be used in conjunction'with a private automatic exchange of the type frequently used to provide telephone facilities for the "employees of a large manufacturing organization or a large office. More particularly, the apparatus is arranged in an improved manner such that two calls may be simultaneously handled at the special or executive substation forming a part of the system, and auxiliary transmitting means and auxiliary receiving means. including an amplifier and a loud speaker may be used in lieu of the regular substation transmitter and receiver in communicating over either of two established connections without releasing either of the two connections.

It is an object of thepresent invention to provide an automatic telephone system wherein two sets of improved and exceedingly simple switching apparatus are utilized for routing connec tions to and from a special substation, and wherein arrangement of the apparatus is such that connections can only be established between the special substation and the other sub- 5. stations of the system.

It is a further object of the invention to provide'improved and exceedingly simple switching apparatus for routing connections from calling ones of a plurality of associated lines to a special substation.

In the illustrated embodiment of the invention there are shown special or executive substation, a plurality of substation lines and means comprising automatic switching equipment controllable only from the special substation for establishing connections between said substation and said lines on calls initiated at said substatin.- More particularly, there is provided in association with the special substation, a plurality of key-operated calling'switches which individually correspond to the substation lines, an automatic switch, and control means operative in response to the operation of any one of the calling switches forcausin'g the automatic switch to select the called line corresponding to the operated calling switch. Calls incoming to the special substation are routed by way of a branch path common to the lines and including a link line through the operation'of auxiliary switchingapparatus which is so connected and arranged that connections'can only be routed by way thereof to thelspecial substation. This auxiliary switching apparatus comprises line and control relays individually associated with the substation lines and "a control relay common to the lines. Each of the control relays includes contacts for conmon "control relay. The common control relay is also operative to cause the operation of a signal device provided at the special substation, thereby ,to announce incoming calls to this substation.

Further features of the invention pertain to the particular arrangement of the apparatus whereby the above ]and additional operating features are attained.

The novel. features believed to be characteristic of the invention are set forth with particularity in the appended claims. The invention, both as to its organization and method of operation, together withv further objects and advantages thereof, will best be understood by reference to the specification, taken in connection with the accompanying'drawings, in which Figures 1 to '7, inclusive, taken together, illustrate a telephone system having incorporated therein the aboveoutlined features of the present invention and Fig. 8 is a diagram illustrating the manner of assembling the other figures of the drawings.

Referring now more particularly to Figs. 1 to '7 of the drawings, there is illustrated a telephone system which may be used to provide exclusive telephone facilities between certain of the oflices of a large office or a manufacturing organization. In brief, the system comprises a special or executive substation including the apparatus shown in Fig. 1 which is arranged to be connected to any one of a plurality of substation lines, two of which are indicated at 1M and 102. The lines of the system terminate in the bank contacts of a line selecting switch 3H] and also in auxiliary switching apparatus indicated generally at and including the relay equipment shown in Fig. '7. The lines extend to substations of the manual type, that associated with the line 10! being diagrammatically illustrated at A and that associated with the line 102 being indicated at B. The apparatus provided at each of the substations A, B, etc., may be of any desired commercial construction and arrangement. In brief, this apparatus may comprise the usual hand set type of transmitter and receiver, a cradle or hook comprising supporting means for the receiver, a cradle or hook switch and a ringer or signal device. Preferably, the elements of each substation circuit are connected and arranged to form the wellknown anti-side-tone circuit, whereby operation of the substation transmitter does not cause operation of the substation receiver.

For the purpose of routing calls from the executive substation including the apparatus shown in Fig. 1 directly to other lines of the system, there is provided automatic switching equipment comprising the above-mentioned rotary switch 3 50. This switch is a standard twenty-five point rotary switch and comprises four sets of bank contacts 35! to M4, inclusive, wipers 315 to M8, inclusive, respectively associated with the enumerated contact sets, and means comprising a motor magnet 310 for driving the enumerated wipers over the contacts of their respective associated sets. As indicated, the contacts in the two sets M3 and 3M terminate the line conductors of the respective substation lines, and the contacts in the set 3|2 terminate the private or control conductors of the lines. The contacts of the fourth contact set 3H terminate conductors extending by way of the cable 295 to the contact springs of a group of coding relays shown in Fig. 2. These conductors individually correspond to the substation lines and are included in marking circuits by means of which operation of the switch 310 is controlled. Since the control apparatus described hereinafter is only arranged to provide facilities for calling twenty substation lines, only twenty of the contacts of each contact set embodied in the switch 3l0 are used, the remaining contacts being left blank. The switch 310 is arranged to be controlled by the group of coding relays shown in Fig. 2 and by the control apparatus 4E5 illustrated in Figs. 4 and 5. More particularly, the coding relays shown in Fig. 2 and indicated at R200, R2l0, R220, R230, R240, R250 and R250, respectively, are arranged to be energized in combinations of three in response to the selective operation of any one of the plurality of calling switches illustrated in Fig. 1. These switches, which are indicated at I00 to 500, respectively, individually correspond to the lines terminating in the bank contacts of the switch 3H0. Each of the enumerated calling switches is of the key operated type and is operable from a normal position to either of two offnorrnal positions individually corresponding to two substation lines. Each of the switches is of the well-known locking type, such that, when once operated to an off-normal position, it remains in its operated position until manually restored to normal. The contact springs of the calling switches are wired in the manner illustrated so that, when any one thereof is operated to one of its two off-normal positions, three of the coding relays shown in Fig. 2 are caused to operate in the manner described in detail hereinafter to complete one of the above-mentioned marking paths extending to one of the contacts in the set 3 of the rotary switch 310, thereby to identify the called line desired by the calling party.

The line wipers 311 and 3l8 of the rotary switch 3l0 are arranged to be connected by way of a link line comprising the conductors C335 and C339 to a repeating transformer 500 through operation of the control apparatus indicated generally at MS. This apparatus comprises a start relay R450 which is arranged to be energized in response to any one of the calling switches shown in Fig. 1 and functions to initiate the operation of the rotary switch 3!!! to select a desired called line. Operation of the rotary switch is controlled by the three relays R420, R430 and R440, of which the relays R430 and R440 directly control the stepping operation of the switch 3|0 and the relay R420 is a test relay for arresting the operation of the rotary switch when its wipers are operated to a position corresponding to a desired called line. The apparatus 415 also includes a ring cutoff relay R550 under the control of a slow-to-operate relay R400, and a back bridge or called party answer relay R5| 0.

As indicated above, the switch 310 and the control apparatus 4l5 associated therewith are controllable only from the executive substation to route connections to the various substation lines terminating at the switch 3l0. For the purpose of extending connections to the executive substation on calls originating at the various other substations, the auxiliary switching apparatus 100 is provided. In brief, this apparatus comprises line relays R1l0, R120, etc., individually associated with the substation lines 10l, 102, etc, control relays R130, R140, etc., individually asso ciated with the substation lines and with the line relays; and a pair of common control relays R100 and R190. Each of the control relays R130, R140, etc., includes contacts for connecting the associated line to a link line including line conductors C18! and C182, respectively, extending to the terminals of one side of a repeating transformer 110. The other side of the repeating transformer 110 is arranged to be connected by way of a line comprising the conductors C186 and C181 to one side of a repeating transformer 300, and the other side of the last-mentioned transformer is, in turn, arranged to be connected by way of a link line comprising the conductors C296 and C291 to the substation apparatus provided at the executive substation. As explained in detail hereinafter, the link line 180, the repeating transformer 110, the line 185, the repeating transformer 300, and the link line 0295, C201, constitute a branch path by way of which connections are extended to the executive substation on calls originating at the other substations. For the purpose of completing a talking loop including this branch path, there is also included in the auxiliary switching apparatus 100, a slow-to-operate called-party-answer relay R150. This latter relay also functions as a ring cut-off relay.

The two sets of switching and control apparatus, namely, the apparatus 100 and the appadenser 525.

ratus M have associated therewithfringing apparatus, not shown, which may be of any conventional arrangement. This ringing apparatus is arranged to be started "into'operation in response tothe application of ground potential to the ringing apparatus start conductor C408 and, during operation, the alternating output voltage thereof is impressed between the ringing current conductor C455 and ground in the usual manner. Any desired source of direct'current having the proper voltage may be used for energizing the various relays and other circuit elements.

Transmitting and receiving means are provided at the executive substation for use in communieating over connections established between this substation and other substations of the system. Calls incoming to this substation are announced through the operation of an alternating current signal device Hill. The transmitting means cornprises a microphone I35, and the receiving means comprises a sound reproducing device in the form of a loud speaker, diagrammatically illustrated at I36/connected in tandeniwith a signal amplifier including the vacuum tube 620. The transmitting and receiving means are interconnected with an anti-side-tone impedance network indicated generally at 630 to form a substation circuit of thewell-known anti-side-tone type having work 631.1 comprises an induction coil or transformer 6! including five windings 6 to 6I5,

inclusive, the last-mentioned of which is coupled to the primary winding MS of an input transformer EH8 having its secondary winding BI'I connected across the non-adjustable terminals of a. voltage dividing resistor EH9. The adjustable terminals of the resistor 5|!) are coupled to the control grid 62I and the cathode 622 of the tube 623). The tube 620,- which is ofthe well-known pentode type and includes a directly heated cathode, iscprovided with anoutput circuit comprising the primary Winding 623 of an output transformer 62'? connected in series with a. source of anode potential between the anode 623 and the cathode 622, For the purpose of biasing the control grid BZI negatively with respect to the cathode 622,, there. is provided a resistor 625 which is commonly included in the output circuit of the tube and the energizing circuit for the cathode 622. The anode current source is .bypassed for signal frequency currents by a con- The secondary winding 629 of the transformer 62? is connected to impress the signal voltage developed thereacross upon the input which the tubetZIl operates, and, hence, withformer I31. a secondary winding I40, one terminal of which voltage is impressed on the input circuit of the amplifier tube 626." The first of the two pal-- ancing networks comprises an adjustable inductance element $3I connected in series with an adjustable resistor 632, and the second network similarly comprises an adjustable inductance element 631 connected inseries with an adjustable resistor 632'. As' shown, the transmitter microphone I35 is included in a substantially closed transmission circuit comprising this microphone, a low impedance condenser I39, and

the primary winding I38 of a coupling trans- This transformer is provided with is connected by way of the conductor CI46 to the junction point between the two transformer windings 6H and BIZ and the other terminal of which is connected by wayof the-conductor CI4'I to, the junction point between the two transformer windings H3 and 6M. The two junction points noted form the conjugate terminals of the anti-side-tone bridge circuit which is effectively formed when the auxiliary'substation equipment is included in an established connection; More particularly, signal currentstraversing the secondary winding, Mil of the transformer I31 during operation of the microphone I35 divide at the two junction points noted, portions of the currents traversing a path comprising the two windings 6H and M3 and one of the two balancing circuits, described above, and other portions of the currents traversing, the two windings M2 and 6M and the established loop circuit extending to the repeating transformer 530 or the remanner indicated, the signal currents respectively traversing the two windings 6H and BIZ are rendered substantially equal in magnitude andopposite in phase at all frequencies within the audio range. Thus, during operation of the transmitter microphone I35, the currents traversingthe windings fil l, fill, M3 and 6M cause substantially equal and opposite signal voltages to be induced in the secondary winding 6|,5, so that substantially no signal energy is delivered to the input circuit of the vacuum tube 52!]. By

virtue of this arrangement, suppression of the side tone is achieved.

For the purposeqof utilizing the transmitting and receiving meansto answer calls incoming to the executive substation and routed by way of the branch path mentioned above, and for the further purpose of permitting the auxiliary transmitting and receiving means selectively to be connected for use in communicating over either of two simultaneously completed connections, each extending to the executive substation, there are provided an answer switch I l I, a transfer switch I i 0, and two control relays R2l0 and R520. Both of the two relays R213 and R520 are arranged to be energized in response to the operation of the answer switch III, the first thereof being operative to condition the signal amplifier comprising the tube for operation, and also to prepare an intermediate loop circuit extending between the repeating transformers 300 and TFO. Arranged to be serially included in the intermediate loop circuit is an adjustable resistor 300 for conducting direct current and by-passed for signal frequency currents by a condenser 305. The relay R210 conditions the signal amplifier for operation by supplying anode and cathode energizing voltages to the tube over circuits traced hereinafter. In this regard. it is pointed out that the tube 020 is of the well-known C-33 type, which type of tube requires only a low anode voltage to provide reasonably good amplification. Hence, the relay energizing source or battery is used to supply anode and screen voltages to this tube. The same source is used to supply energizing current to the cathode 022 of the tube 020, a voltage dropping resistor being serially included in the 'hode energizing circuit for the purpose of fixing the resistance of the circuit so that the rated current of the cathode heater traverses the circuit.

The relay R520, which is energized in response to operation of the answer switch I I I, and deenergized in response to the subsequent operation of the transfer switch H0, is arranged to transfer the linl; line C515, C5753, leading to the input terminals 603 and 604, from a connection with the wii .ing 50! f the repeating transformer 500 to a co' motion with the winding 150i of the repeati...g transformer 000. This relay also functions selectively to include the two balancing circuits, described previously, in the anti-sidetone impedance network 030 in the manner pointed out more in detail hereinafter. In order to remind a user of the executive substation equipment that the answer switch HI is in its operated or off-normal position, there is provided no.1 lamp I20 which is arranged to be energized in response to the operation of the switch 2 I! to its oif-normal position.

For the purpose of preventing an established con ction extending to one of the subordinate ens from being reestablished after the r.lease operation is performed at the subordinate substation and before the connection is released at the executive substation, there are included in the control apparatus 6- 5 two relays and the first of which is of the slowe perate type, and the last-mentioned of which More particularly,

. "1 for energizing the transmitter microiI-l'i is normally supplied to this microover a circuit including the winding of When. however, a release operation is d at a subordinate substation involved relay R450.

tion of the switching equipment to establish a connection to one of the substations on a call initiated at the executive substation and assuming that the substation A is being called, the call is initiated by operating the calling switch I to the particular off-normal position corresponding to the called substation. More particularly, if the substation A is being called, the key switch I00 is operated from its normal position to its lower off-normal position to close the springs of the three sets H4, H and H6. As indicated above, three of the coding relays illustrated in Fig. 2 respond to the operation of any one of the key switches to an off-normal position to provide a marking path for marking the particular called line in the contact level 3H of the rotary switch 340. In the case under consideration, when the springs II i, I I5 and H6 are closed, circuits are completed for energizing the three relays R200, R2 50 and R220, which circuits commonly include the winding of the start relay R450. The operating circuits for the three relays noted may be considered as a single circuit extending from battery at the lower terminal of the winding of Ri0 by way of this winding to C290, where it divides; one branch extending by way of the winding of R220, CI32 and the springs I I4 to the grounded conductor CIE'I; a second branch extending by way of R2I0, CI33 and the springs I IE to the grounded conductor CIZ'I; and the third branch extending by way of R200, CI34 and the springs II5 to the grounded conductor CI2'I. When energized over the above-traced circuits, the four relays R200,R2 I 0, R220 and R450 operate and the first three thereof function to complete a marking path identifying the line 322 as the line being called. This marking path extends from ground at RAZOI by way of RA2I I, RA22I, and C21! to the twenty-fifth contact of the contact set 3| I. Due to the above-described arrangement of the coding relays, none of the other marking paths individually corresponding to the other lines directly accessible to the special equipment are completed.

As a further example of the manner in which the coding relays operate in response to selective operation of the calling switches, it is pointed out that if the switch I00 is operated to close the contact springs H2, H3 and III a series-parallel circuit is completed for energizing the three coding relays R200, R2 I 0 and R230, and the start In this case the two relays R200 and R2I0 are energized over circuits substantially similar to the branch circuits traced above and the relay R230 is energized over a branch circuit extending by way of C293, the winding of R230, CI3I and the contact spring III to the grounded conductor CI2I. When the three relays R200, R2I0 and R230 operate, a marking path is completed for applying ground to the twenty-fourth contact of the set 3| I, this path extending from ground at RA20I by way of RAZII, RAZS! and C218 to the twenty-fourth of the set 3I I.

Again, if the tenth calling switch I09 is operated to its lower off-normal position, the Springs I2I, I22 and I23 are closed to complete a seriesparallel circuit for energizing the three relays R200, R240 and R260 in series with the relay R450. In this case the relay R200 is energized over a branch circuit extending by way of CI34 and the springs I23 to the grounded conductor CIZI; the relay R240 is energized over a branch circuit extending by way of CI30 and the springs I22 to the grounded conductor CI2I; and the relay R260 is energized over a branch circuit extending by way of Cl28 and the springs I2I to the grounded conductor Cl2'l. Upon operating,the three relays complete a path extending from ground at RA26I and by way of RA246, RA203 and C219 for impressing marking' ground on the nineteenth contact of the contact set 31 l. It will be apparent from the foregoing description that by selectively operating the calling switches, the coding relays may be caused to operate in different combinations of three, thereby selectively to complete the twenty marking paths extending to the respective contacts of the contact set 3| I.

In any case when three of the coding relays are energized the start relay R450 is also energized and, upon operating, completes, at RA453, paths commonly including the grounded armature RA559 and C410 for applying ground to the ringing apparatus start conductor C408, thereby to initiate the operation of the ringing apparatus in the usual manner. At RA454, the relay R450 applies ground to the amplifier start conductor C294, thereby to condition the amplifier comprising the tube 320 for operation. More particularly, operation of the relay R450 results in the connection of the positive terminal of the current supply source or battery to C294, so that anode potential is impressed through the primary winding 628 of the output transformer 02'! on the anode 623 of the vacuum tube 620. Operation of the relay R450 also-results in the completion of a circuit for energizing the oathode 622 of the tube 620, this circuit extending from the grounded positive terminal of the supply battery at RA454 by way of C294, the voltage dropping resistor 624, the cathode 622 and the biasing resistor 625 to the negative terminal of battery. It will be noted that'positive screen potential is applied to the screen electrode of the tube 620 directly from the positive terminal of the supply battery over the path including the conductor C294.

At RA45I, the relay R450 completes a circuit for energizing'the slow-to-operate relay R430, this circuit extending from ground at RA45I by way of RA422, RC423, C336, the armature 3| 9, C331 and the winding of R430 to battery. The relay R430 operates, after an interval, and, at RA43I, prepares a circuit, traced hereinafter, for energizing the relay R420 when the wipers of the rotary switch 3l0 are stepped to the position correspondingto the selected called line 10L At RA435, the relay R430 completes a circuit for energizing R440, this circuit extending from ground at RA45I by Way of RA422, RC423, C336, the armature 3l9', C331, RA435 and the winding of R440 to battery.

When energized over the circuit just traced, the relay R440 operates to complete, at RA44I, a circuit for energizing the motor magnet 3l9 of the switch 3I0, this circuit extending from ground at the grounded contact R0421 by way of RA426, RA44I, C338, and the Winding of the magnet 3|9 to battery. Upon operating, the magnet 3I9 steps the wipers M5 to '3I8, inclusive, of the switch 3 I 0 one step and interrupts, at its associated armature 3l9, the abovetraced operating circuits for the two relays R430 and R440. The relay R440 immediately re- .stores to interrupt, at RA44I, the operating circuit as traced above for the magnet 3), whereupon this magnet restores its associated armature 3H3 to recomplete the operating circuits for the two relays R430 and R440, respectively. When the operatin'gcircuit for R440 is again completed,

this relay operates to recomplete the above-traced circuit for energizing the magnet 3H3, causing this magnet to again step the enumerated wipers of the switch 3i0 a second step and to again interrupt the operating circuits for the two relays R430 and R440. The above-described interaction between the relay R440 and the magnet 3H3 continues until suchtime as the wipers of the switch 3i0 are stepped to the position corresponding to the called line FBI, or until the 'iper 3H5 is moved into engagement with the contact marked with ground over the abovetraced marking path. During the interaction between the relay R440 and the magnet 3E9, the relay R430, due to its slow-to-release characteristic, stays operated.

When the wiper 3E5 of the switch M0 is operated into engagement with its associated twenty-fifth contact, the above-mentioned circuit is completed for energizing the relay R420, this circuit extending from the grounded conductor C211 by way of the twenty-fifth contact of the set 3H, the wiper 3I5, C333, RA 431 and the lower winding of R420 to battery. When its lower winding is energized over the circuit, just traced, the relay R420 immediately operates to interrupt, at RA426 and R0421, the above-traced circuit for energizing the magnet 3w, thereby to arrest the interaction between this magnet and the relay R440 and to prevent further movement of the Wipers embodied in the rotary switch 3l0. At RA425, the relay R420 completes a holding circuit for itself, this holding circuit extending from the grounded conductor C333, by way of RA425 and the upper winding of R420 to battery. At RA422 and R0423, the relay R420 interrupts a point in the circuits over which R430 and R440 are being held energized, causing these two relays to restore. At RA422 and RC424, the relay R420 prepares a holding circuit, traced hereinafter, for R560;

As indicated above, the auxiliary switching apparatus shown in Fig. 7 is only operable to extend connections to the executive substation and can not be used to set up communication connections between the other substations. Also, the switching equipment and associated control apparatus shown in Figs. 2 to 5, inclusive, can only be used to forward connections from the executive substation to the other substations. Thus, the provision of apparatus for testing the idle or busy condition of a selected called line is unnecessary.

A short time interval after the switch 310 is operated to the position corresponding to the line 10! or, more particularly, a short time interval after the relay R420 operates, the slow-to-release relay R430 restores to complete, at RA432 and RC434, a circuit for'energizing the slow-to-operate control relay R400, this circuit extending from ground at RA45I by way of the winding of R400, R0434, RA432, RA42I, C334, the wiper 3I6 and its associated twenty-fifth contact to battery as impressed upon the control conductor C101 of the line 10! through the lower winding of R'H0. Shortly following its energization over the circuit just traced, the slow-to-operate relay R400 operates to complete, at RA40I, a circuit for energizing the ringing relay R560, this circuit extending from ground at RA45I by way of RA422, RC424, RA40I, C463 and the winding of R560 to battery. Upon operating, the relay R560 completes, at RA561, a holding circuit for itself, which circuit extends from ground at RAM)! .75

by way of RA tZZ, R0424, C452, RASBI and the winding of R550 to battery. At RA558, the relay R560, upon operating, applies ground to the control conductor 0101 of the called line 10I, thereby to complete a circuit for energizing the lower winding of the line relay R1I0 associated with the called line 10L Specifically, the abovementioned circuit extends from ground at RA508 by way of 0054, RA42I, 0334, the wiper 3I6 and its associated twenty-fifth contact, 0101 and the lower winding of R1I0 to battery.

When its 'c-wer winding is energized over the circuit just traced, the relay R1I0 operates to interrupt, at RAII I and R01I2, a point in the operating circuit, traced hereinafter, for R130. At RA1I5 and R01I1, the relay R1I0 opens a point in the operating circuit for R160, thereby to prevent the latter relay from operating when the call is answered. At RA1I5 and RA1IS, the rela R'IIO opens two points in its primary operating circuit, traced hereinafter, and in so doing clears the line 10! of the impedance of its own windings. At RA1I8, the relay R1I0 completes an obvious circuit for energizing the slowto-release relay R190, which latter relay operates to open, at RA19I, a point in the common portion of the primary operating circuits for all of the line relays. Thus, calls are prevented from being forwarded to the executive substation at a time when this substation is busy.

It will be noted that, when R550 completes the above-traced circuit for R'ZIE], it also completes a path for short-circuiting the winding of R400, causing the last-mentioned relay to restore. At RAI, the relay R400, upon restoring, opens a point in the operating circuit for R550, but the latter relay is held operated over its holding circuit as traced above. At RA555 and its associated grounded contact R0500, the ringing relay R550 prepares a locking circuit, traced herein after, for the slow-acting ring cutoff relay R550. At RA56I and RA562, the relay R550 completes a path for projecting ringing current over the called line 10I, thereby to energize the signal device at this substation. Specifically, the path traversed by the ringing current extends from one terminal of the ringing current source, not shown, by way of C405, 0461, RA551, RA55I, 0330, the wiper M8 and its associated twenty-fifth contact, the line conductor 0104, the signal device at the called substation, the line conductor 0103, the wiper 3H, C335, RA553, RA562, RA552, R0553 and the lower winding of R550 to the other terminal of the ringing current source. The resulting operation of the signal device provided at the called substation indicates that this substation is being called. The relay R560 also completes a path for transmitting ringback-tone energy to the input circuit of the signal amplifier tube 620. Specifically, the path of the ring-backtone current extends from the ungrounded terminal of the ringing current source, not shown, by way of the conductor 0405, the condenser 403, 0468, RA509, RA550, RA542, R0529, RA528, 0515, the transformer windings (H2 and SI I, the inductance element 63E, the resistor 032, 0512, R0522, RA52I, 0514, the transformer windings BI3 and 5I4, 0515, RA524, R0525 and the upper half of the winding to the grounded terminal of the busy-tone source at the grounded center tap 502 of the winding 50I. This current causes an induced voltage to be developed in the transformer winding 0I5 which is impressed through the coupling transformer 6I8 between the input electrodes of the tube 620, and the amplified output energy is delivered through the transformer 621 and the volume control network I4I to the loud speaker I35 for reproduction. The reproduced ring-back-tone signal indicates to the calling party that the desired substation is being rung.

The desired conversational connection is completed when the call is answered at the called substation A. More particularly, when the receiver at the substation A is removed from its hook, a direct current bridge is placed across the line conductors 0103 and 0104 of the line TN to complete a circuit substantially similar to that, traced previously and traversed by the ringing current, for energizing the lower winding of the ring cutoff relay R550. When its lower winding is energized, the relay R550 first closes the X contacts associated with RAG, thereby to complete the above-mentioned holding circuit for itself, this circuit extending from ground at R0565 by way of RA555 and the two windings of R550 in series to battery. At RA552 and RA551 and their respective associated resting contacts, the relay R550 opens two points in the abovetraced path for projecting ringing current over the selected called line 10I and, in so doing, opens two points in its own operating circuit. At these same armatures and their respective working contacts, the relay R550 extends the called substation loop circuit to the repeating transformer 500, thereby to complete a circuit including this loop circuit for energizing the back bridge or called party answer relay R5I0. At RA558, the relay R550 opens a point in the above-traced path traversed by the ring-back-tone current and, at RA559, it opens a point in the abovetraced path for applying ground to the ringing apparatus start conductor 0408.

When the relay R5I0 operates, it completes, at RA5II and R05! 3, a circuit for energizing the slow-to-release relay R540, this circuit extending by way of RA532, 051I, the transmitter microphone I35, the winding I38, C510 and through the winding of R540 to battery. Current for energizing the transmitter microphone I35 is conducted to the microphone over the circuit just traced. Upon operating, the relay R540 prepares, at RA54I, a circuit, traced hereinafter, for energizing R530. At RA542, the relay R540 opens a further point in a path for conducting ring-backtone current by way of the anti-side-tone impedance network 530 to the input circuit of the amplifier tube 620.

Following the operation of R540, the desired conversational circuit is completely established, talking battery being supplied to the loop extending to the called substation A through the windings cf the back bridge relay R 5| 0, and talking battery being supplied to the transmitter microphone I35 over the circuit, traced above, for energizing the relay R540. Signal currents incoming to the special substation and traversing the windings 503 and 504 of the repeating transformer 500 cause induced voltages to be developed in the winding 50I coupled to the two windings 503 and 504, whereby signal currents corresponding to the incoming signal currents are caused to flow through the anti-side-tone impedance network 630. More particularly, the path followed by currents traversing the winding 50I may be traced as extending from the lower terminal of this winding by way of R0529, RA520, 0510, the two windings 6I2 and SI I, the inductance element 63L the resistor 032, 0512, R0522, RA52I, 0514, the two windings H3 and 614, 0515, RA524, RC525, backto the upper terminal-of the winding 50L The resulting induced voltages developed in the winding 6I'5 are im-- pressed through the couplingtransiormer BIB and the voltage dividing resistor 0I9 between the input electrodes of the amplifier tube 620, thereby to cause corresponding amplified currents to flow in the output circuit of this tube. Such currents cause induced'voltages to be produced in the winding 020 of the output coupling transformer 021, which voltages are impressed through the potentiometer network I4I on the input terminals of the loud speaker I36 for reproduction.

Audio frequencycur'rents developed by operation of the microphone I ,are causedto circulate in the closed low impedance transmission circuit comprising the primary winding I38 of the transformer I31 and the condenser I39. Such currents cause corresponding induced voltages to be developed in the secondary winding I40 of the transformer I3'I which are impressed by means of the two conductors CI46 and CM! between.

opposite terminals of the anti-sidetone impedance network 630. More particularly, signal terminals of the anti-side-tone impedance network and follow the two paths traced previously, whereby the well-known anti-side-tone efiect is realized. 1

The release of the established communication connection is primarily under the control of the calling party. If the called party at the substation A restores his receiver to its hook before the calling key switch I00 is returned to normah'the established called party loop circuit extending to the repeating transformer 500 is interrupted,

causing the back bridge relay R5I0 to restore in the usual manner. Upon restoring, the relay R5I0 interrupts, at RA5I I and RC5I3, the abovetraced circuit for energizing R540 and, at RA5II and. RC5I2, it completes a circuit for energizing the slow-to-operate relay R530, the latter circuit extending. by way of RA52'I, RA54I and the winding of R530 to battery. Upon operating, the

.relay R530 completes, at RA53I, a holding circuitfor itself, this circuit extending from ground at RA555 by way of RA53I and the winding of R530 to battery. It is noted that the relay R530, although of the slow-to-operate type, operates to complete its holding circuit before the slow-torelease relay R500 restores to. open the operating circuit forR530. At RA532, the relay R530, upon operating, opens a further point in the abovetraced operating circuit for R540 and, in so doing, opens a further point in the circuit by way of which energizing current is delivered to the microphone I35. At RA534, the relay R530 applies ground by way of C410 to the start conductor C408, thereby to reinitiate the operation -'of..the ringing apparatus providing this apparatus is not already in operation. At RA533, the relay R530 0 plfies an alternative path for impressing ringask-tone voltage upon the input circuit of the signal amplifier tube 020, thereby to cause ring-back tone to be reproduced by the loud speaker I30. The ring-back-tone signal prouced by operation of the loud speaker I30 indicates to the calling party thatthe key switch I00 has not been restored to normal.

The purpose of providing the relay R530, which operates in response to the release operation performed'at the called substation, isthat of positively preventing eavesdropping on the part of a called party, following the termination of a conversation between the called party and the currents traversing the winding I40 divideat the.

calling executive. Thus, in the absence of this relay and its associated circuits, the called party at the substation A could, after hanging up, again remove his receiver from its hookto recomplete the talking connection, provided the operated key switch I 00 was not restored to normal by the calling party. In this regard, it will be appreciated that the back bridge relay REI 0 would respond to the recompletion or" the loop circuit to again complete the circuit for delivering energizing current to the transmitter I35. Obviously, with this connection reestablished, the called party could listen to any conversation being carried on in the office of the executive. With the arrangement illustrated, however, the relay R500, upon operating, is held operated independently of the back bridge relay R5I0 and, in its operated position, holds interrupted, at RA532, the above-traced path by way of which energizing current is delivered to the transmitter microphone I35. Thus, the channel for transmitting signal currents outgoing from the special substation to the called substation is rendered completely inactive and eavesdroppingon'the partofa called party is positively prevented. I

As. indicated above, the common control apparatus M5 is released in response to the restoration of the operated key switch Q00 to its normal position. When this switch is returnedto normal, the above-traced operating circuits for the four relays R200, R2l0, R2120 and R050 are interrupted, causing all of these relays to restore. 'When' the first three of the enumerated four relays restore, the above-traced" markingpath extending to the twenty-fifth contact of the contact set 3H in the switch M0 is interrupted. When this path is interrupted, the holding circuit for R420 is broken, causing this relay to restore. Thus, the two operated relays R020 and R450 are caused to restore. When the relay R420 restores, it interrupts, at RAdiiZ and R0420, the circuit over which R500 is'being held energized. At RA42I, the relay R420 opens a point in the above-traced circuit for energizing the line relay R'IIO, causing this line relay and the relay R190 sequentially to restore in the order named, thereby to establish the circuit connections normally prevailing when the apparatus 100 is idle; During the restoration of the relay R500, the con-, tact springs RA503 and R0504 are moved into engagement before the loop circuit extending to the called substation is interrupted, providing the called party has not restored hisreceiver to its hook. Whenthe enumerated springs are moved into engagement, the condenser 543 is connected through RA 55I between the line conductors C330 and C335. This condenser absorbs the voltage transient which occurs when the talking 'loop is interrupted at RAI following the restoration of R560 completely to normal and thus prevents the transient from being reproduced as an objectionable click by the'receiver at the called substation. When R500 falls back, it interrupts, at RA565 and RC560, a point in the holding circuit for the ring cutofirelay R550, whereby the lastmentioned relay is caused to restore. The relay R560 also opens at RAI a point in the path con-- necting the condenser 543 between the link line conductors C335 and 0339. Upon restoring, the. relay R550 opens, at RA555, a point in the holding circuit for R530. If R530 is operated, it restores to interrupt, at RA533, a point in the abovementioned alternative path for delivering ringback-tone energy to the signal amplifiertube 020.

ground from C410. Following the restoration of R530, the common control apparatus M5 is entirely released.

From the foregoing description, it will be apparent that connections may be routed from the executive substation by way of the switch 3I0 to any one of the various substation lines of the system by selectively operating the calling keys illustrated in Fig. 1. As a further example, it is briefly pointed out that, if the calling key I00 is operated to its upper off-normal position to cause the operation of the three coding relays R200, R2 I0 and R230, a marking path extending to the twenty-iourth contact of the set SII embodied in the switch 3I0 is completed which identifies the line 102 as the called line. The manner in which the switch Sic, the control apparatus 4 I 5, and the line relay R120, associated with the called line 102, respond and forward the connection to the line 102 is exactly the same as that just described with reference to the connection extended to the line 10I.

As indicated previously, the apparatus shown in Fig. '1 is only operative to extend a connection from a calling one of the lines 10!, 102, etc., to the executive sub-station. A connection may be routed by way of this apparatus to the executive substation only at a time when the executive substation is idle. If, for example, a person at the substation B initiates a call to the executive substation by lifting the hand set at this substation from its supporting hook or cradle, the hook or cradle switch operates to connect a direct current bridge between the line conductors C105 and C106, thus completing the primary operating circuit for the relay R120. Specifically, this circuit extends from ground at R0121 by way of RA126, C105, the bridge connected between C105 and C105 at the calling substation, C105, RA125, the upper winding of R and RAlSl to battery. When its upper winding is energized over this circuit, the relay R120 only partially operates to complete at the X contacts associated with RA124 a circuit for energizing the slow-to-operate control relay R140. The last-mentioned circuit extends from ground at R0122 by way of RA12I, RA124, and the winding of R to battery. At the X contacts associated with RA124', the relay R120 prepares a point in its secondary operating circuit, traced hereinafter. Upon operating, the relay R140 attracts its associated armatures RAMI and RA142 into engagement with their respective associated working contacts, thereby to extend the calling loop to the repeating transformer 110 and to complete a circuit for energizing the common control relay R180. The circuit for energizing R160 may be traced as extending from ground by way of the winding 112, C182, RA1 l2, C105, the bridge across C105 and C105 at the calling substation, C105, RA14I, C181, the winding 11! and the winding of R160 to bat" tery. When energized over this circuit, the relay R operates to complete, at RA10I, the abovementioned secondary operating circuit for R120. which circuit extends from ground by way of RA16i, RA124 and the lower winding of R120 to battery. It will be noted that the relay R150 also prepares the secondary operating circuits for the respective other line relays. Thus. at RA164, the relay R prepares a secondary operating circuit for R1I0. The secondary operating circuit for the relay R110 is, however, open at RA1I4.

When the secondary operating circuit for the .relay R1I0 is completed, this relay completes its operation to interrupt, at RA'IZS'and RA126, two points in its primary operating circuit as traced above. At RA12l and RC123, the relay R120 completes a holding circuit for R140, this circuit extending from ground by way of RA162, RC123, RA12I, RA124, and the winding of R140 to battery. At RA128, the relay R120, upon fully operating, completes an obvious circuit for energizing the slow-to-release relay, R190, which latter relay operates to interrupt, at RA19i, a point in the common portion of the primary operating circuits for the line relays respectively associated with the various substation lines, thereby to prevent any iurther ones of the line relays from being energized in response to the initiation of calls on the lines respectively associated therewith.

At RA1$6 and its associated working contact, the relay R150, upon operating, completes a path extending from ground at R0156 for applying ground to the ringing apparatus start conductor C 100, thereby to initiate the operation of this apparatus in the usual manner. At RA and its associated working contact, the relay R160 completes a circuit for energizing the signal device IBG provided at the executive substation, this circuit extending from one terminal of the ringing current source, not shown, by way of C405, RA165, RA15I, C186, the signal device 560, C181, RA152 and through the winding of R150 to the opposite terminal of the ringing current source. A portion of the ringing current is by-passed through the condenser 151 and the winding 113, thereby to cause a ring-back-tone voltage to be impressed upon the loop extending to the calling substation B. The resulting operation of the receiver at the calling substation indicates that the executive substation is being rung.

To answer the call, the answer switch III is operated to its off-normal position to close the springs 125' and thereby complete a circuit for energizing the control relay R520. The circuit noted extends from the grounded conductor CI21 by Way Of the springs I25, the springs I24, CI52, and the winding of R520 to battery. At the springs I25, a circuit is also completed for energizing the signal lamp I20 in series with the winding of R210, this circuit extending from the grounded conductor Ci21 by way of the springs 125, the signal lamp I26, CI21' and the winding of R210 to battery. When energized over the above-traced circuit, the relay R210 operates and the signal lamp I25 is illuminated to indicate that the answer switch III occupies its off-normal position. The relay R210, upon operating, completes, at RA212, a path for applying ground to the amplifier start conductor C294, whereby th signal amplifier comprising the vacuum tube 620 is conditioned for operation in the manner described previously. At RA21I, the relay R210 completes a direct current bridging path between the line conductors C186 and C131 of the link line 185, which path includes the two windings 303 and 304 of the repeating transformer 300. More particularly, the bridging path noted extends from the line conductor C181 by way of the winding 304, the resistor 300, C298, RA21I, C239 and the winding 303 to the opposite line conductor C106.

When the above-traced path between the conductors of the line 185 is completed, the slow-tooperate relay R150 is energized by direct current and operates. The operating circuit for R150 extends from ground by way of the ringing current source, not shown, C405, RA165, RA15I, C186, the above-traced bridging path, C181, RA152 and calling substation.

the winding of R150 to battery. Upon operating, the relay R150 completes, at RAI and RA152 and their respective associated working contacts, a holding circuit for itself, whichcircuit includes the two windings 113 and 114, and the-closed loop extending to the transformer 300.. At RA15I and R 152, the relay R150 opens two points in its operating circuit and in so doing interrupts the circuit for energizing the signal. device Hi0 and the circuit for impressing ring back-tone energy on the loop extending to the At RA153, the relay R150 prepares an alternative path for projecting ringing current over the line 185 and, at RA154, it

interrupts a point in the path for applyingground to the ringing apparatus start conductor ferent, it is desirable to p'rovidealternative line this relay operates to complete, at RA528', the

abovetraced circuit for energizing R540 and by way of which energizingcurrent is delivered to the transmitter microphone i35. At RA521, the relay R520 opens a point in the above-traced operating circuit for R530, thereby to prevent the last-mentioned relay from operatinginresponse tothe operation of R500. At RA524 and RA528 and'their respective associated resting contacts R0525 and R0520, the relay R520 disconnects the line conductors C515 and C516 from the winding 5th of the repeating coil500, and; at

these same armatures and their respective as sociated working contacts RC520 and RC529, connects the line conductors C515 and C51 6'by way of "the linkline conductors C296 and C291, respectively, to the opposite" terminals ,of the repeating transformer winding l.

newly established talking connection in lieu'of the repeating transformer 500, which lattertransformer is used only on calls originated 'at the,

special substation and routed by way'of the switch tit. Since the impedance characteristics of the line apparatus associated with the re- 7 pea-ting transformers 5630 and 300 may be difbalancing circuits arranged alternatively to be included in the anti-side-tone impedance network 530. As described above, with the relay R5 20 Ideenergized, the line balancing circuit comprising the inductance element 63! and the resistor 032 is operatively included in the anti-side-tone impedance network 530. When, however, the relay R520 operates, it substitutes the other balancing circuit, namely, that comprising the inductance.

element 133! and the resistor 032, for the firstrnentioned balancing circuit in the anti-side-tone impedance network. This is accomplished through the operation of R520 to disconnect, at

RAEZ l, the conductor C514, extending to the upper terminal of the winding 6| 3, from the conductor C512 and to connect this conductor to the conductor C513. It will beunderstood that the line balancing circuit comprising the inductance 1 element 63! and the resistor 632' is adjusted to have afrequency-impedance characteristic substantially matching that of the equivalent impedance of the repeating transformer 300 and theline impedances associated therewith as detern lined at the line terminals 043 and 644 of the Following the operation of l R150, the talking loop is extended to Thusthe re-- peating transformer 300 is incorporated in the impedance network 630. Similarly, the line .ba'lancing circuit comprising the inductance element 63] and'the resistor E'32.is adjusted to have a frequency-impedance characteristic substantially the same as that of theequivalent impedance of the repeating transformer 500 and the line 1mpedances, associated therewith as determined -at the line'terniinals 643 and 644 of the network 530.

Following the operation of R520, the desired conversational connection is fully completed. After the conversation is terminated the connection is only released when the answer .switch Hi is restored to normal and the calling party hangs. up. When the calling party hangs up, the bridge across the line conductors C and C106 is interrupted to open the circuit over which R160 is being held energized. When R100 restores it opens at RAiiil and RAMA-respectively, the holding circuits for R and R causing the two last-mentioned relays to restore. Upon restoring, the relay'R120 opens the operating: circuit for R100, which latter relay restores after an interval. When R100 restores it .reprepares, at RA10I, .the operating circuits for all of the line relays. It. is noted that the two relays R120 and R140 are connected and arranged to restore before the relay R100 falls back. Thus;

the relay R120 has ample time to restore to open,

at RA'EM, its secondary operating circuit and, at RA124, the holding circuit forRMil before the relay R130 can reoperate in response to a waiting call to recomplete the two last-mentioned circuits. By this arrangementthepossi bility of two substation lines being simultaneously and inadvertently connected. to the link line 100 is positively precluded. It is noted further that the relay R is arranged to restore considerably before the relay R120 fallsback, thus preventing the relay R1130 from momentarily reoperating, i

Assuming the answer switch H1! is still in its operated position .at the time the relay R1960 :re-

stores, this relay completes, at RA160. and its associated restingicontact, the. above-mentioned path extending byway of RA forapplying ground to the ringing apparatus start conductor C400. AtRA105 and its associated resting contact, the relay R completes a path for projecting ringing current over the loop extending "to the executive substation, this path extending by way of the condenser 158' .and RA153- The ringing current energy; delivered over the loop to the input circuit of the tube 020 is amplified in this tube and delivered to the loud speaker I30 for reproduction, thereby to indicateto the called party at the executive substation that the answer switch I I l still occupies its offenormal I position. It is noted that the capacitance value of the condenser 150 is chosen of a value such that the reproduced signal is not objectionably loud, but is clearly audible.

When the answer switch Hi is retored to normal, the. above-traced operating circuit for the relay R520 is interrupted to cause "this relay and the relay R540 to restore in the order named, 7

and the series circuit for energizing the signal lamp I25 and the relay R210 is interrupted acausing the. last-mentioned relay torestore. When R210 restores, it removes ground from the amplifier start conductor C204, thereby to render the signal amplifier inactive, and opens, at RA21I, a. point in the above-traced bridging path between the conductors C106 and C181 .of the line 185. Following the restoration of the three relays R520, R540 and R210, the transmitting and receiving means are entirely disconnected circuit for the relay R520 is interrupted at the from the link line C295, C201. When the direct current bridge between the conductors of the line 185 is interrupted, the relay R150 is deenergized and restores. Upon restoring, this relay interrupts, at RA153, the above-traced path for projecting ringing current over the line 185, and, at RA155, it interrupts the above-mentioned path for applying ground to the start conductor C408.

If the called party at the executive substation restores the answer switch III to normal before the connection is cleared out at the calling substation B, the relays R520, R550, R210 and R150 are caused to restore in the manner just described. Under this condition the relay R is still operated, so that the relay R150, upon restoring, recompletes the above-traced path for applying ground to the start conductor C408 and the above-traced circuit for again energizing the signal device I00 provided at the executive substation. When thereafter the calling party hangs up the relays R100, R100, R and R190 are caused to restore in the order named and in the manner described previously. The relay R160, upon restoring removes ground from the start conductor C408 and opens, at RA165, the circuit for energizing the signal device I50. Thus, the connection is entirely released.

The mode of operation of the relays R and R190, common to the lines, and of the line and control relays individual to another of the lines to extend a connection to the executive substation is substantially the same as that just described and will readily be apparent from the preceding description.

As indicated in the introductory portion of the specification, a party at the executive substation may utilize the automatic switching apparatus including the switch 310 to route a connection to one of the substation lines during the course of a conversation over an established connection extending to another of the substation lines, as* suming of course that the first connection is routed by way of the apparatus 100. For example, if a party at the executive substation answers the above-described call initiated at the substation B and desires to secure information from a party having an office in which the substation A is located, the direct call to the substation A is initiated by first operating the calling switch I00 to the lower off-normal position in the manner described above. The coding relays R200, R2I0 and R220, the common equipment M5 and the rotary switch 3I0 respond to the operation of the switch I00 to its cit-normal position by performing the previously described functions, whereby a connection is completed between the repeating transformer 500 and the called line 10I. In this regard it will be recalled that the three relays R210, R520 and R540 are operated. Hence, the amplifier comprising the vacuum tube 620 is conditioned for operation and the line conductors C515 and C516 incoming to the anti-side-tone impedance network 030 are connected through to the repeating coil 300. The operated condition of the three relays R210, R520 and R540 in no way interferes with the operation of the other relays included in the common control apparatus 4l5 to cause the connection to the desired line 10l to be established. When the transfer switch H0 is operated to its off-normal position, following operation of the calling switch I00 to the off-normal position corresponding to the called line 10I, the operating springs i24, causing this relay to restore. When the relay R520.falls back, it disconnects the line conductors C515 and C515 from across the terminals of the transformer winding 30I and connects these line conductors to the respective terminals of the transformer winding 50L thereby operatively to include the transmitting and receiving means comprising the loud speaker I36 and the transmitter microphone I35 in the established connection extending to the called substation line 10L The relay R520, upon restoring, also disconnects the conductor 0510 from the conductor C513 and connects it to the conductor C512, thus substituting the line balancing circuit comprising the inductance element E3I and the resistor 032 for the balancing circuit comprising the inductance element 63I' and the resistor 032 in the anti-side-tone impedance network 630. With the relay R520 in its restored position, the original circuit for supplying energizing current to the microphone I35 and for holding R540 operated, is interrupted at RA528'. At this time, however, the back bridge relay R5I0 is in its operated position so that a multiple circuit extending from groimd at RAEII and by way of RC5I3 and RA532 is completed for maintaining R540 operated and for supplying energizing current to the microphone I35.

It will be noted that to efiect the transfer operation just described the transfer switch H0 is operated to its off-normal position without restoring the answer switch III to its normal position. So long as the last-mentioned switch is retained in its off-normal position, the abovetraced circuit by way of which the relay R210 is energized is held completed and, so long as R210 is energized and remains in its operated position, the loop circuit extending to the transformer 110 and including the line is main tained completed at RA21I. After the two connections extending, respectively, to the substations A and B are established, the party at the executive substation may, by operating the transfer switch IIO between its normal and off-normal positions, converse over either of the two connections at will. Thus, if the party at the executive substation, after conversing with the called party at the substation A, desires to again converse with the party at the substation B, he may do so by returning the transfer switch IIO to its normal position to recomplete the above traced operating circuit for R520. It will be apparent, in view of the foregoing description, that when R520 reoperates it substitutes the balancing circuit comprising the inductance element 63I' and the resistor 032 for the balancing circuit 03I; 032 in the antiside-tone impedance network, disconnects the line conductors C515 and C513 from the transformer winding 50!, and reconnects the line conductors C515 and C516 to the terminals of the repeating transformer winding 30l.

In order to release the two established connections, it is only necessary for the two parties at the substations A and B to hang up and for the party at the executive substation to restore the operated switches I00 and III to normal. When the switch I00 is returned to its normal position, the three operated coding relays R200, R2) and R220 are caused to restore, together with the start relay R450. Following the restoration of these four relays, the remaining operated relays embodied in the common control apparatus are caused sequentially-to restore in the manner described in detail in a preceding portion of the specification. Also, when the. answer switch II I is returned toits normal position, the three operated relays R520, R540 and R210 are deenergized and restore in the manner described previously. When the two parties at the substations A and B, respectively; hang up the apparatus 100 is restored to normal in the manner described previously. l

With the arrangement illustrated it is not possible for a connection .to beextended by way of the apparatus 100 to the executive substation at a time when the executive substation is busy. This is true for the reason that in any case when a connection is established,'the relay R190 is operated and holds open, at RA'I9I, a point in the common portion of, the respective primary operating circuits for the line relays R110, R120, etc.,

and with their primary operating circuits open the line relays cannot be energized in response to the initiation of calls on their respective associated lines.

While there has been described what is at present considered to be the preferred embodiment of the invention, it will be understood that various modifications may be made therein, and it-is intended to cover in the appended claims all such modifications as fall within the true spirit and scope of the invention.

What is claimed I I 1. In a telephone system, ,a substation, a plurality of telephone lines, means comprising automatic switching equipment controllable from said substation for establishing connections between said substation and said lines on calls initiated at said substation, and auxiliary automatic switching apparatus associated with said lines, said auxiliary switching apparatus being so connected and arranged that a talking connection is extended from a calling one of saidlines to said substation over a route from which said automatic switching equipment is excluded in re-' sponse to the initiation. of a call on said one line and that connections can only be routed by way thereof to said substation.

2. In a telephone system, a first substation, a plurality of substation l'ines each including a pair of line conductors, substations individually associated with said lines: and each including means for connecting a bridge across the line conductors of the associated line, means comprising automatic switching equipment controllable from said first substation for establishing connections between said first substation and said lines on calls initiated at said first substation, and auxiliary switching apparatus associated with said lines, said auxiliary switching apparatus being so connected and arranged that a connection is extended from a calling one of said lines to said first substation in response to the connection of a bridge across the conductors of said one line and that connections can only be routed by way thereof to said first substation.

3. In a telephone system, a first substation, a

ated with said lines, said auxiliary switching apparatus including a control relaycommonly associated with saidlines and at least one relay individual to each line and being so connected and arranged that connections can only be routed by Way. thereof to-said first substation, said control relay and the relay individual to a calling one of said lines being operative in response to the connection of a bridge across the conductors of said one line to extend a connection from said one line .to said first substation.

4.. In a telephone .system, a first substation, a plurality of substationjlines each including a pair of line conductors, substations individually associated with said lines and each including meansfor connecting a bridge across the line conductorsof the associated line, means comprising automatic. switching equipment controllable from said first substation for establishing connections between said first substation and said lines on calls initiated at said first substation, auxiliary switching. apparatus associated with said lines, said auxiliary switching apparatus including a relay common to said lines and at least one relay individual to each line and being so connected and arranged that connections can only be routed by way thereof to said first substation, .a signal device at said first substation, a normally incomplete circuit for energizing said signal device, said common relay and the relay individualto a calling one of said lines being operative in response to the connection of a bridge across the conductors of said one line to extend a connection from said one line to said first substation, and means responsive to the operation of said common relay for complet ing said circuit.

5. In a telephone system, a substation, a plurality of telephonelines, means comprising automatic switching equipment contr'ollablefrom said substation for establishing connections between said substation and said lines on calls initiated at said substation, auxiliary switching apparatus associated with said lines, said auxiliary switching apparatus being so connected and arranged that a connection is extended from a calling one of said lines to .saidsubstation in response to the initiation of-a call on said one line and that connections can only be routed by way thereof to said substation, and means operative in response to the initiation of a call on one of said lines for rendering said auxiliary switching apparatus ineffective, to extend a connection from any of the other lines. to said substation.

6. In a telephone system, a first substation, a

plurality of substation lines each including a pair of line conductors, substations individually associated with said lines and each including means'for connecting abridge across the line conductors of the associated line, means comprising automatic switching equipment controllable from said first substation for establishing connections between said first substation and saidlines 'on calls initiated at said first substation, auxiliary switching apparatus associated with said lines, 'said'auxiliary switching apparatus being sozconnected and arranged that a connection is extended from a calling one of saidlines to said first substation in response to the connection of a bridge acrossthe conductors of said one line and-that connections can only be routed by way thereof to said first substation, and 'means operative in response to the ing apparatus from operating to extend a connection from a second line to said first substation in response to the connection of a bridge across the conductors of said second line.

7. In a telephone system, a first substation, a plurality of substation lines each including a pair of line conductors, substations individually associated with said lines and each including means for connecting a bridge across the line conductors of the associated line, means comprising automatic switching equipment controllable from said first substation for establishing connections between said first substation and said lines .on calls initiated at said first substation, auxiliary switching apparatus associated with said lines, said auxiliary switching apparatus including a control relay commonly. associated with said lines and at least one relay individual to each line and being so connected and arranged that connections can only be routed by way thereof to said first substation, said control relay and the relay individual to a calling one of said lines being operative in response to the connection of a bridge across the conductors of said one line to extend a connection from said one line to said first substation, and means operative in response to the operation of the relay individual to said one line for preventing any of the other similar relays from operating in response to the connection of bridges across the conductors of the respective associated lines.

8. In a telephone system, a first substation, a plurality of substation lines each including a pair of line conductors, substations individually associated with said lines and each including means for connecting a bridge across the line conductors of the associated line, means comprising automatic switching equipment controllable from said first substation for establishing connections between said first substation and said lines on calls initiated at said first substation, auxiliary switching apparatus associated with said lines, said auxiliary switching apparatus including a relay common to said lines and at least one relay individual to each line and being so connected and arranged that connections can only be routed by way thereof to said first substation, a signal device at said first substation, a normally incomplete circuit for energizing said signal device, said common relay and the relay individual to a calling one of said lines being operative in response to the connection of a bridge across the conductors of said one line to extend a connection from said one line to said first substation, means responsive to the operation of said common relay for completing said circuit, and means operative in response to the operation of the relay individual to said one line for preventing any of the other similar relays ,from operating in response to the connection of bridges across the conductors of the respective associated lines.

9. In a telephone system, a substation, a plurality of telephone lines, an automatic switch having access to said lines, a plurality of manually operable calling switches at said substation, said calling switches individually corresponding to said lines, means operative in response to the operation of any one of said calling switches for causing said automatic switch to select the called line corresponding to said one calling switch and for establishing a connection between the called line and said substation, a branch path common to said lines and arranged to be included only in connections routed by way of said lines to said substation, said branch path including a link line, line relays individually associated with said telephone lines and individually operative in response to the initiation of calls on the respective associated lines, and control relays individually associated with said telephone lines and said line relays, each of said control relays'including contacts for connecting the associated line to said link line, thereby to extend a connection from the associated line to said substation, and having an operating circuit arranged to be completed in response to operation of the associated line relay.

10. In a telephone system, a substation, a plurality of telephone lines, an automatic switch having access to said lines, a plurality of manually operable calling switches at said substation. said calling switches individually corresponding to said lines, means operative in response to the operation of any one of said calling switches for causing said automatic switch to select the called line corresponding to said one calling switch and for establishing a connection between the called line and said substation, a branch path common to said lines and arranged to be included only in connections routed by way of said lines to said substation, said branch path including a link line, line relays individually associated with said telephone lines and individually operative in response to the initiation of calls onthe respective associated lines, control relays individually associated with said telephone lines and said line relays, each of said control relays including contacts for connecting the associated line to said link line, thereby to extend a connection from the associated line to said substation, and having an operating circuit arranged to be completed in response to operation of the associated line relay, and means operative in response to the initiation of a call on any one of said telephone lines for preventing the line relays, other than the line relay associated with the calling line, from operating in response to calls initiated on the respective associated lines.

11. In a telephone system, a substation, a plurality of telephone lines, an automatic switch having access to said lines, a plurality of manually operable calling switches at said substation, said calling switches individually corresponding to said lines, means operative in response to the operation of any one of said calling switches for causing said automatic switch to select the called line corresponding to said one calling switch and for establishing a connection between the called line and said substation, a branch path common to said lines and arranged to be included only in connections routed by way of said lines to said substation, said branch path including a link line, line relays individually associated with said telephone lines and individually operative in response to the initiation of calls on the respective associated lines, control relays individually associated with said telephone lines and said line relays, each of said .control relays including contacts for connecting the associated line to said link line, thereby to extend a connection from the associated line to said substation, and having an operating circuit arranged to be completed in response to operation of the associated line relay, and means operative in response to the operation of any one of said line relays for preventing the other line relays from operating in response to calls initiated on the respective associated lines.

12, In a telephone system, a substat1on,.a pluto said lines and arranged to be included only in connections routed by way of said lines to said substation, said branch path including a'link line, line relays individually associated with said telephone lines and individually operative in re-- sponse to the initiation of calls on the respective associated lines, control relays individually/associated with said telephone lines and said line re-' lays, each of said control relays including contacts for connecting the associated line to said link line, thereby to extend a connection from the associated line to said substation, and having an. operating circuit arranged to be completed in response to operation of the associated line relay, a signal device at said substation, a normally incomplete circuit for energizing said signal device, and means responsive to the operation of any one of said line relays for completingsaid circuit.

13. In a telephone system, a substation, a plurality of telephone lines, an automatic switch having access to said lines, a plurality of manually operable calling switches at said substation,

said calling switches individually corresponding to said lines, means operative in response to the operation ofv any one of said calling switches for causing said automatic switch to select the called line corresponding to said one calling switch and 7 associated lines, control relays individually associated with said telephone lines and said line relays, each of said control relays including contacts for connecting the associated line to said link line, thereby to extend a connection from the associated line to said substation, and having an operating circuit arranged to be completed in response to operation of the associated line relay, a signal device at said substation, a normally incomplete circuit for energizing said signal device, means responsive to, the operation of any one of said line relays for completing said circuit, and means operative in response to the operation of anyone of said linerelays for preventing the other line relays from operating in, response to calls initiated on the respective associated lines; 14. In a telephone system, a first substation,

a plurality of substation lines each including a pair of line conductors, substations individually associated with said lines and each including at said first substation, a branch path common to said lines and arranged to be included in connections routed by way of said lines to said sub-'. station, said branch path including a link line,

75 line relays individually associated with said lines,

control relays individually associated with said lines and said line relays, each of said control relays including contacts for connecting the associated line to said link line, thereby to extend a connection'from the associated line to said first substation, an additional relay common to said lines, each of said line relays having an operating circuit including the line conductors of the associated line and arranged to be completed in response to the connection of a bridge across the line conductors of the associated line, operatingand holding circuits for each of said control relays, means responsive to the operation of one of said line relays for completing the operating circuit of the one control relay associated with said one line relay, said additional relay having an operating circuit arranged to be completed in response to the operationof any one of said control relays and including the line conductors of any calling one of said lines, holding circuits for each of said line relays, means responsive to the operation of said additional relay for completing the holding circuit for said one control relay and said one line relay and means responsive to the operation of said line relay for interrupting the operating'circuits for all of the other line relays.

15. In a telephone system, a first substation, a plurality of substation lines each including a pair of line conductors, substations individually-associated with said lines and each including means for connecting a bridge across the conductors of the associated line, automatic switching equip-' ment controllable from said iirst substation for establishing connections between said first sub station and said lines on calls initiated at said first substation, a: branch path common to said lines and arranged to be included in connections routed by way of said lines to said substation, said branch path including a link line, line relays individually associated with said lines,

control relays individually associated with said lines and said line relays, each of said control relays includingcontacts for connecting the associated line to said link line, thereby to extend a connection from the associated line to said first substation, an additional relay common to said lines, each of said-line relays having a primary operating circuit including the conductors of the associated line and being partially, operative in response to the completion of its primary operating circuit, each of said primary operating circuits being arranged to be completed in response to the connection of a bridge across the conductors of the associated line, secondary operating circuits for said line relays, operating and holding circuits for each of said control relays, said additional relay having an operating circuit including the conductors of anycalling one of said lines, means responsive to the partial operation of one of said line relays for completing the operating circuit for the associated control relay, means responsive to the operationof said associated control relay for completing the operating circuit for said additional relay, means responsive to the operation of said additional relay for completing the secondary operating circuit for said one line relay, thereby to cause said one line relay to complete its operation,

means responsive to the last-mentioned operation of said one line relay for completing the holdingcircuit ,for said associated control relay,

iii

lit

and means responsive to the last-mentioned operation of said one line relay for interrupting 

